The present invention relates to a radio base station and to a method pertaining to a radio communications system in which the radio base stations are equipped with antenna arrays whose antenna lobes can be aimed in desired directions. Such antennas can be used in the known GSM system, for instance.
The use of adaptive antennas, also called antenna arrays, that can be controlled to aim the antenna lobes in desired directions is well known to the art. It is also well known to make a directional estimate of the direction-of-arrival of a received signal and to control the antenna array on the basis of this estimate. This has long been the practice in military radio communications systems, to enhance the interference tolerance of the system. The use of antenna arrays in civilian radio communications systems, and then in mobile communications systems in particular, has attracted much attention in recent years. In mobile communications systems, the radio frequency must be used extremely effectively, in order to satisfy capacity and quality requirements.
A radio base station of a mobile communications system serves mobile stations located in the cell of the base station with radio communication. The radio base station has access to a plurality of radio channels to this end. The radio channels are also used in another cell at a distance from the first cell given by the frequency reuse factor. When these connections are setup over the same radio channel, interference will occur between radio connections setup in the first cell and those setup in the second cell. The minimum frequency reuse factor is determined by the maximum permitted interference. A small frequency reuse factor enables effective use of the frequency spectra.
The use of adaptive antennas in radio base stations enables interference between different radio connections to be reduced, and therewith provides a potential for reducing the frequency reuse factor while maintaining the quality of the radio connection.
Patent Application WO 96/22662 discloses a radio communications system in which the radio base stations are equipped with antenna arrays and means for making a directional estimate of a received signal. WO 96/22662 addresses the problem of establishing communication in a cell with several terminals over one and the same radio channel. Radio connection with each of these terminals can be selected with the aid of spatial information, i.e. a directional estimate for each terminal. Spatial separation, however, requires the terminals that use the same radio channel to be spatially scattered, i.e. to be located in different directions from the radio base station. When a new connection is to be setup with a terminal, a radio channel is allocated for the connection. The allocated radio channel may not be used by another terminal in the same direction. In order to make allocation of a suitable radio channel possible, the radio base station is equipped with a database that includes a list of all available radio channels in the radio base station and for each of the radio channels a directional estimate for each of the mobile stations that uses the radio channel.
The problem addressed by WO 96/22622 is different than the problem addressed by the present invention, namely the use of only one radio channel for communication with several radio terminals that are served by one and the same radio base station. The solution proposed in WO 96/22662 also assumes that the radio base station knows the identity of a mobile station that uses a given channel.
The present invention addresses the problem of needing to make a new estimate of the direction to the mobile station each time a radio connection with a mobile station is switched to a new radio channel, before the antenna lobe can be aimed towards the mobile station. This means that when beginning to use the new radio channel, the signal-disturbance ratio is considerably poorer than if it were possible to aim the antenna lobe immediately in an advantageous manner.
When setting-up a new traffic connection, the signalling messages are sent over different radio channels. These messages require good connection quality, in order for the connections to be setup quickly and correctly. Furthermore, signalling will preferably seize signalling channels over a short time period, in order for the signalling channel to have a good capacity.
A change of radio channel for an existing connection can also be made if the radio quality of the first channel is poor. If this new channel does not immediately improve the quality of the radio transmission, due to the absence of a directional estimate, there is a risk that the connection will be broken.
In the case of radio communications systems, such as the known GSM system, in which a radio base station solely puts through signalling information from the mobile network to the mobile station, and vice versa, a further problem is one of being able to use the directional estimate in a channel change without needing to change the interface between the radio base station and the mobile network.
Accordingly, in respect of a radio communications system whose radio base stations are equipped with directional antenna lobes, an object of the present invention is to enable the antenna lobe to be directed in respect of a radio channel immediately a radio connection is moved to this channel.
This problem is solved in accordance with the invention with a method in which an estimate of the direction of the mobile station is generated when the connection takes place over a first physical radio channel. A mobile station recognition character is also identified when the connection is setup over the first radio channel. The recognition character and the directional estimate derived from the recognition character are registered in a list. When a command relating to channel activity is received via the fixed connection of the radio base station, a recognition character for the mobile station for which the new physical radio channel shall be used is also identified. The list is read when the allocated channel is activated, and the directional estimate is found on the basis of the mobile station recognition character. This directional estimate is used to direct the antenna lobe in an appropriate direction, immediately use of the new channel is commenced.
The present invention also relates to a radio base station that includes means for carrying out the aforesaid method. The radio base station is typically equipped with at least one, normally several, transceiver units. Each of these transceiver units handles a radio carrier-wave, which in TDMA systems is divided into a number of physical radio channels. Transceiver units convert in uplink a physical radio connection to a bit stream that is forwarded to the radio network over a fixed connection, and vice versa in downlink. In accordance with the invention, these known transceiver units are provided with means for reading certain of the signalling messages that are exchanged between the mobile station and the radio network and therewith identify the mobile station recognition character and detect messages to the effect that the connection has been switched to a new radio channel. The radio base station also includes a database having connections to each transceiver unit in said station. A transceiver unit registers in the database directional estimates for a mobile station with respect to an identified mobile station recognition character and possibly a new channel connection when such a connection is identified. The transceiver unit includes means for finding such a directional estimate in the database, provided that this directional estimate and the new channel have been registered. The transceiver unit then uses the directional estimate to shape the antenna lobe in a beneficial fashion.
When a radio connection is moved to a new radio channel, the invention has the advantage of enabling an antenna lobe belonging to the new channel to be shaped immediately in an appropriate direction. This improves the transmission quality of important signalling that is transmitted immediately after the change of channel.
Because the antenna lobe is shaped advantageously from the very beginning, interference on other radio traffic is also reduced.
The invention also has the advantage of enabling the directional estimate to be generated when communication takes place over a beacon channel. The frequency reuse factor in respect of a beacon channel is greater than in the case of standard traffic channels, so that interference will be lower in respect of the important signalling that takes place in the beacon channel. This enables a channel estimate of better quality to be made in a beacon channel than in a standard traffic channel.
The invention will now be described in more detail with reference to preferred embodiments thereof and also with reference to the accompanying drawings.